This invention pertains to arrangements for producing negative ions and in particular to negative ion sources suitable for use with mass spectrometers and other instruments having significantly large minimum ion velocity requirements.
In 1938, Bleakney and Hipple reported a positive ion cycloidal mass spectrometer (Physics Review, 53 (1938) 521) which utilized crossed electric and magnetic fields to separate positively charged particles into a mass spectrum. A major problem with this type of instrument has proven to be the attendant decrease in ion velocity, which causes difficulties in focusing the instrument. As a result, the cycloidal mass spectrometer has been largely abandoned in favor of other mass spectrometer arrangements.
Conceptually, negative ion mass spectrometers are also fraught with problems. One particular problem has been that of providing a suitable source of negative ions in which the ionizing electron stream is prevented from being merged with the end-product negative ions, during acceleration of those negative ions into the mass spectrometer. Generally, a gas source is subjected to an electron stream in a way that enhances electron capture by the particles. One way to insure such enhancement is to limit the electron stream to thermal velocities. However, even the most efficient capture rates are of the order of less than one percent, and the uncaptured 99-plus percent of the electron stream comprises possible candidates for acceleration within the negative ion accelerator stage. One major problem with negative ion instruments has been the inability to prevent the introduction of electrons into the negative ion stream.
The capture of an electron by most gaseous molecules to form negative ions is a favored process only when the electron has near-thermal kinetic energy, except for a few molecular species which have resonances at intermediate energies. Heretofore, thermalization has generally been accomplished by accelerating an electron from a hot filament into a relatively high pressure gas, wherein the electron loses kinetic energy by multiple collisions. The drawback to this technique, when utilized with mass spectrometers and the like, is the required differential of a high pressure in the ion source and a low pressure in the analyzer sections of the mass spectrometer. This requires extensive differential pumping. The method of cooling electrons, that has been used almost excusively to date is entitled "Negative Chemical Ionization" (NCI). In this method, negative ions produced in a high pressure gas source are introduced into an analyzer which typically operates at a high vacuum, thereby requiring a pressure drop at the source-analyzer interface of about eight orders of magnitude. This pressure differential causes major problems which can only be partially overcome at great expense. A method for thermalizing electrons that is more readily compatible with high vacuum instruments would be desirable for certain types of negative ion studies, since there are a wide range of electronegative molecules that are best analyzed as negative ions.
A recently published method by J. E. Delmore (inventor of the present invention) [International Journal of Mass Spectrometry Ion Physics, 43 (1982) 71] uses electrons as they are emitted from a hot filament, for electron capture studies with SF.sub.6. The minimum emitter temperature at which a minimum usable electron current of a few microamperes can be generated is 1300 K, so even at these conditions the electrons are more energetic than might be desired. Also, in this method, the electrons spend little time at this minimum energy, prior to being accelerated by the lens, and the hot filament can cause molecular decompositions that yield ion fragments at the effective ionization region of the source. A method for reducing the kinetic energy of an electron to less than that at which it is emitted, at a location within the ionization region of the ion source (which is some distance from the emitter) would be advantageous. Simple retardation lenses are not suitable at the 0.05-0.20 eV levels corresponding to an electron temperature of several hundred degrees centigrade, due to space charge buildup.
It is therefore an object of the present invention to provide a source of negative ions in which a supply gas is made to capture electrons.
Another object of the present invention of the present invention is to provide a negative ion source which preferentially accelerates negative ions without accelerating the ionizing electrons.
Yet another object of the present invention is to provide a negative ion source in which ionizing electrons are prevented from being appreciably accelerated when placed in an accelerating field which extracts the negative ions thus formed.
Another object of the present invention is to provide a negative ion source in which appreciable residence times for ionizing electrons in thermal energy ranges is realized.
Yet another object of the present invention is to provide an arrangement for reducing the kinetic energy of electrons emitted from a hot filament to energy levels less than that of the emission energy, and in regions where particle ionization takes place.
Still another object of the present invention is to provide a negative ion source which ionizes gaseous particles at a pressure roughly equivalent to the operating pressure of downstream instruments which utilize such negative ions.
These and other objects of the present invention are provided in a negative ion source in which an elongated strip-like electron emitting filament is located at right angles to a downstream draw-out electrode. A gas stream to be ionized passes adjacent to the filament, and through an aperture in the draw-out electrode. The draw-out electrode is energized at a positive potential, relative to the filament. Thus, an electric field is directed from the filament toward the draw-out electrode. A shield electrode is located adjacent the filament, and extends generally parallel thereto. The gas stream passes between the filament and the shield electrode, in an ionization region of the ion source. A magnetic field placed within the ionization region extends perpendicular to the electric field, in a direction generally parallel to the axis of the filament. Electrons emitted from the filament travel through the ionization in cycloidal orbits, in a direction mutually orthogonal to both the electric and magnetic fields. The electrons are slowed down to thermal energy levels while traversing the cycloidal orbits, a feature which enhances electron capture by the gas stream. The resulting negative ions are preferentially accelerated out of the ionization region, leaving unaccelerated electrons to remain in the ionization region.
In a method of the present invention, negative ions are produced for use in a negative ion instrument which includes an ion lens having a focus and a focal axis. An electric field, aligned parallel to the focal axis, is provided. Also provided is a magnetic field aligned perpendicular to the electric field. An input gas is made to flow past an electron emitter, toward the ion lens. The emitter is energized, causing electrons to be emitted therefrom in cycloidal orbits, traveling in a direction mutually orthogonal to the electric and magnetic fields. The input gas is directed across the cycloidal orbits of the electrons such that the electrons are captured by the input gas, producing negative ions. Thereafter, negative ions are preferentially accelerated toward the ion lens, causing the electrons to remain behind.